1. Field of the Invention
The present invention relates to a work mounting apparatus for mounting an IC chip or some other electronic component, as a work, on a printed board.
2. Description of the Related Art
A work mounting apparatus of this type is disclosed in Published Unexamined Japanese Patent Application No. 62-114290, for example. This conventional apparatus comprises a rotating table and a plurality of suction heads arranged at predetermined intervals on the outer peripheral edge of the table. Each suction head is capable of up-and-down motion.
A workbench is disposed under the rotating table, and a work supply section and a work mounting section are arranged on the workbench at a distance from each other in the circumferential direction of the table. A plurality of work feeders are radially arranged in the supply section, while a printed board is disposed in the mounting section.
When one of the suction heads is positioned in one of the work feeders of the supply section as the table rotates, the rotation of the table is stopped. In this state, the suction head is lowered, receives a work from the work feeder by suction, and is then raised to its original position. When the rotation of the table is restarted, thereafter, the suction head is transferred from the supply section to the mounting section. When the suction head is positioned in the mounting section, the rotation of the table is stopped again. In this state, the suction head is lowered again, whereupon the work is released from the suction, so that it is mounted on the printed board in the mounting section.
According to the work mounting apparatus described above, the rotation of the table must be stopped every time the suction head is move up or down, that is, every time the work is received or mounted in the supply or mounting section. In other words, the rotation of the table and the operation of the suction head are performed intermittently, so that the tact time cannot be shortened, and therefore, the work mounting efficiency is low.
In these circumstances, there is a demand for continuous-type work mounting apparatuses in which works can be received and mounted without stopping the rotation of the table.
In one such continuous-type mounting apparatus, for example, a continuously rotatable drum, which is used in place of the rotating table, is surrounded by a plurality of work heads which repeat up-and-down motion as they turn. More specifically, the work heads turn around a turning axis which extends in the radial direction of the rotating drum, and their respective postures are kept fixed during the turning motion.
Thus, if a suction nozzle is provided protruding downward from each work head, it can repeat up-and-down motion while moving in the circumferential direction of the rotating drum, as the drum rotates and as the work head turns.
If the turning speed and turning direction of the work head are properly set with respect to the peripheral speed of the rotating drum, moreover, the relative speed of the suction nozzle with respect to the supply and mounting sections, that is, the ground speed, can be reduced substantially to zero when the suction nozzle lowers so that the supply and mounting sections come closest to each other. Even while the rotating drum is being continuously rotated, therefore, the suction nozzle of the work head can securely receive a work from one of the work feeders of the supply section by suction, and then mount the received work head on a printed board in the mounting section.
In the above continuous-type work mounting apparatus, however, if the up-and-down motion of the suction nozzle in association with the rotation of the drum, i.e., the cycloid motion of the work head, is unstable, the work cannot be accurately mounted in the target mounting position on the printed board, or even the mounting of the work in the mounting position cannot be carried out.
On the other hand, to permit the works to be taken out from different work feeders, the cycle of the cycloid motion need by variable.
Thus, the cycloid motion of the work head must be controlled with high accuracy, and to this end, stable turning of the work head is essential.
The work head, however, has a relatively large mass, and the turning speed thereof is liable to change during one turning motion.
Further, the turning shaft of the work head is usually connected to the drive source by means of gear train, and therefore, backlash of the meshed gears is unavoidable. If the backlash is large, undesirable variation in the rotation of the turning shaft occurs, which is a major cause of instability of the turning speed of the work head.
A belt-type planetary mechanism, for example, may be employed to keep the posture of the work head fixed during the turn of the work head. More specifically, such a planetary mechanism may comprise a toothed pulley provided at the outer periphery of the turning shaft, a toothed pulley arranged coaxially with the axis of the work head, and a toothed belt passed around and between these pulleys. If the planetary mechanism of this type is used, the rotation of the work head is restrained by the toothed belt during the turn of the work head, and consequently, the posture of the work head can be kept fixed.
The work mounting apparatus is further provided with correction means for correcting the posture of the work after the work is attracted to the suction nozzle, and this correction means is indispensable in order to accurately mount the work on the printed board.
In the case of the above planetary mechanism, however, it is difficult to continually maintain the same state of engagement of the toothed belt with the paired pulleys during the turn of the work head. Namely, the teeth of the pulleys and belts are subject to pitch error when they are manufactured. Thus, a slight variation occurs in the posture of the work head during the turning motion, adversely affecting the correction of the work posture by the correction means.